[Solved] The mother's immune system may have a defensive response to substances produced by male fetuses. This is most likely to contribute to A) erotic plasticity. B) refractory periods. C) sexually transmitted infections. D) the fraternal birth-order effect. Fixed Costs: Costs that do not change with the level of goods or services produced within a certain scale. Product: Any item or service that is created through a process and is intended to be supplied to a market to satisfy a want or need. Fixed Costs: Expenses that do not change in total despite fluctuations in the volume of goods or services produced. Sunk Costs: Costs that have already been incurred and cannot be recovered or changed, which should not influence future business decisions.